Due to their refractory and brittle nature, ceramics are materials that are difficult to form.
The current processing means are relatively long and expensive, when highly refractory ceramics such as carbides or nitrides have to be sintered.
It is generally difficult to manufacture parts of complex shape with ceramics, and in particular refractory ceramics.
It is therefore often preferable to manufacture structures from ceramic elements or parts of simple shape, then to join these elements to produce the final structure, of more complex shape.
Due to the high temperatures, for example in the vicinity of 1000° C. and above, used in the applications of ceramics such as silicon carbide, the joining of these ceramics by bonding with organic products is excluded.
Moreover, the conventional techniques of joining by welding using a beam of energy with or without solder (TIG, electron or laser welding) and involving a partial melting of the parts to be joined cannot be used for joining ceramics.
This is because during local heating the thermal shock will lead to a sudden rupture of the ceramic before a possible start of melting.
Consequently, solid phase diffusion welding, sinter-joining, and reactive brazing are currently the most common techniques for producing refractory ceramic joints.
Solid phase diffusion welding and also sinter-joining have the disadvantage of being restrictive from the point of view of their implementation.
For solid phase diffusion welding, the shape of the parts must remain simple if uniaxial pressing is used, or else require complex tooling and preparation comprising, for example, manufacture of an envelope, leaktight sealing under vacuum, hot isostatic pressing and final machining of the envelope, if HIP (hot isostatic pressing) is used.
In the case of sinter-joining, the same problems remain (shape of the parts, complexity of implementation) with, in addition, the need to control the sintering of a solder powder to be inserted between the two materials to be joined.
These two techniques also require the use of holds, plateaus of long duration (one to several hours) at high temperature since the procedures used involve solid state diffusion; these long durations could favour the enlargement of the grains of refractory alloys and embrittle them.
Reactive brazing is an inexpensive, easy-to-use technique that is currently the most commonly used. Parts of complex shape can be produced by carrying out capillary brazing and the operations are limited to placing the solder between or in the vicinity of the joint and melting the braze.
Thus, at the present time, the best known process for joining refractory ceramics such as SiC is the process known as the “BRASIC” process of the Commissariat à I'Energie Atomique [Atomic Energy Commission].
This process, described in particular in French patent application nos. FR-A-2 749 787, FR-A-2 748 471, FR-A-2 728 561, and FR-A-2 707 196, makes it possible to join refractory ceramic parts, for example parts made of silicon carbide, by refractory brazing.
The parts to be joined are brought into contact with an intermetallic braze and the assembly formed by the parts and the braze is heated at a brazing temperature equal to the melting point of the braze in order to form a refractory joint. This process, which has been developed on an industrial scale, however has certain limitations:                i) the operating temperature of the brazed assembly must not exceed the melting point of the braze;        ii) the braze corresponds to a second phase that may, in some cases, prove unacceptable for the use of the material, and may, for example, cause the neutronic incompatibility of the braze;        iii) the nature of the braze is intrinsically linked to the nature of the material to be brazed, that is to say for each ceramic to be assembled it is necessary to develop a new type of specific braze, which may result in a long and costly development period.        
Furthermore, Spark Plasma Sintering (SPS) technology is known. The first patent applications relating to this technology were filed by K. INOUE at the end of the 1960s. These patent application matured into U.S. Pat. Nos. 3,241,956 and 3,250,892.
But it was necessary to wait until the end of the 1990s for the SPS technique to enjoy an experimental boom.
SPS is a sintering technique that consists in simultaneously applying to the bulk sample or pulverulent sample to be densified, or to the parts to be joined, a uniaxial pressure and current pulses of high intensity.
The powders or parts may be made of metal, ceramics or polymers.
The rise in temperature is applied to the sample via an assembly of plates and pistons made of graphite, the powder is inserted inside a graphite pelleting press. The assembly consisting of the pelleting press, the pistons and the plates is the sole assembly in the vacuum chamber to rise in temperature.
More precisely, the operating principle of an SPS machine and its main components is represented in FIG. 1. The powder (1) is placed in a graphite sleeve (2), between two pistons (3). A pressure (4) is applied to these pistons (3), and a direct current (5) is applied to electrodes (6). The powder (1), the pistons (3), the graphite sleeve (2) and a portion of the electrodes (6) are placed inside a vacuum chamber (7). Instead of the powder, it is possible to place two ceramic parts to be joined between the two pistons so as to have in the die the succession: piston-1st ceramic-2nd ceramic-piston.
The temperature is monitored via an optical pyrometer that also controls the electric power injected into the assembly. The currents used during the sintering may range up to 8000 A.
The main advantage of the SPS technology is the possibility of densifying the samples in relatively short time periods of the order of a few minutes, for example 5 to 10 minutes.
The sintering rapidity often makes it possible to minimize the grain growth and to attain, for certain materials, a density close to 100%.
However, spark plasma sintering (SPS) has been applied little or not at all to refractory ceramics and has not been applied to the joining of two refractory ceramic parts.
U.S. Pat. No. 6,515,250 relates to a process and a device for joining parts in which, in a first step, the surfaces of the parts to be assembled are brought into contact without using a graphite die. Then a pulsed current or a combination of a pulsed current and a direct current is applied to said surfaces while applying a pressure to the parts so as to temporarily join or connect the parts (“Step 1”).
Next, a heat treatment is carried out on the temporarily joined parts in order to make this joining permanent and to obtain a joint strength equivalent to the strength of the material of each of the joined parts.
It is specified that the process of this patent makes it possible to join metal parts together and also a metal part to a non-metal part or non-metal parts together.
In the examples from this document, the joining of bars made of identical materials, for example made of stainless steel (SUS 304), or made of alloy tool steel (SKD 61) is carried out.
It is moreover indicated that the joining may be carried out between metal parts made of different materials, for example, between a part made of SKD 61 and a part made of aluminium alloy, between a part made of SUS 304 and a part made of SUS 420J2, between a part made of copper alloy and a part made of SUS 420J2, and between a part made of SKH 51 (high speed tool steel) and a part made of SKD 61, and also between a part made of metal and a non-metal part, or between non-metal parts, without using welding materials or brazing metals.
It is also specified that when this process is used for joining parts made of different materials, for example metal, and ceramic or plastic, a part that has gradient characteristics, that is to say that has a gradual variation of its properties from one side of the part to the other, is placed between the parts to be joined.
Finally, it is mentioned that the process of this patent may be applied to “various types of joining”, for example the joining of an ultra-hard metal and a normal metal, or the joining of a part made of aluminium and a corrosion-resistant and wear-resistant part.
In conclusion, this patent neither describes nor suggests the joining together of two parts made of ceramics, and even less of two parts made of refractory ceramics, and does not mention the particular problems posed by the joining of two ceramics, in particular of two refractory ceramics.
Furthermore, the joining (step 1) is carried out without a graphite die. However, since ceramics are not good electrical conductors, it is impossible for the pulses of current to pass through the ceramics to be joined. This first step can therefore only function for metals or more generally current-conducting materials.
Moreover, the joining by pulsed current that is similar to SPS only constitutes an intermediate step leading to a temporary joining, the heat treatment following this step is absolutely necessary for ensuring the good cohesion of the interface.
In fact, it may be considered that in the process of this document the first step only constitutes a simple positioning of the parts and that no true joining is obtained.
U.S. Pat. No. 6,384,365 describes a process of consolidating or repairing turbine component parts, such as turbine blades by the spark plasma sintering (SPS) technique.
The blade may be made of a conventional cobalt superalloy or of a single crystal nickel-based superalloy.
If the blades are covered with a base coat made of, for example, MCrAlY, and/or with an outer thermal barrier ceramic coating such as a stabilized zirconia coating, this coating should be completely removed. It is specified that when the opposing surfaces to be joined of the turbine part are made of a ceramic material, a ceramic powder is then placed between these opposing surfaces. No exemplary embodiment is given, in particular concerning the joining of two parts made of ceramic, in particular made of refractory ceramic.
The SPS treatment time is less than 5 minutes at a temperature below 1500° C.
In conclusion, in this patent, in order to join ceramics it is obligatory to use a third interlayer phase as in the brazing processes described above.
Moreover, this patent does not describe or mention the joining of refractory ceramics, because the temperature of 1300° C. is insufficient for joining refractory ceramics, for example of carbide or nitride type.
In view of the aforegoing there is therefore a need for a process for joining ceramic parts by spark plasma sintering (SPS) that makes it possible to join refractory ceramic parts, namely having a sintering temperature above 1900° C.
There is also a need for such a process that allows the joining of these parts in a simple, reliable, rapid, inexpensive manner and that makes it possible to obtain a bond or joint of great cohesion and great strength.